The invention is related to protecting splices of optical fibers, in particular to heating a heat-shrinkable tube in order to make the tube shrink around a splice of optical fibers which is to be protected, and more particularly to an assembly including a heat-shrinkable tube intended to be used for protecting splices of optical fibers.
Optical fibers are presently widely used for communicating information such as in large telecommunication systems, primarily owing to their large reliability, their insensitivity to electrical interference and their high capacity. In order to facilitate handling optical fibers, they are often provided as fiber ribbons, in which a plurality of optical fibres are located adjacent to each other, optically isolated from each other and from the environment, but in some manner fixed in parallel to each other, for example by means of a polymer coating. Such a ribbon usually contains 4, 6, 8 or 12 parallel fibers.
When splicing two different optical fibers, which is usually performed by welding, the polymer coatings of the fibers must first be removed. Thus, both the individual coating of the optical fibers and the material, which secures the fibers to each other to form a fiber ribbon, must be removed. Thereupon the uncovered or bare fibres are cut in order to thereafter be welded to each other.
Heat-shrinkable tubes are often used for protecting such splices of optical single fibers and of optical ribbon fibers, which are used in the telecommunication field. Then such a tube is applied over an end of a single fiber or of an optical fiber cable such as a ribbon fiber, and then the cable is spliced to another cable. Finally the heat-shrinkable tube is moved to cover the bare portions of the optical fibers and then heated making it shrink and be firmly attached around the bare portions and to the splice. When heating the tube it must be carefully observed that no air remains inside the shrinking tube since such remaining air degrades the mechanical supporting and protecting function of the tube.
When heating the protection tubes often ovens are used having a U-shape. Also resistance wires applied inside the tube have been proposed. Thus, in U.S. Pat. No. 4,509,820 a protective packaging assembly for a spliced portion of optical fibers is disclosed, the assembly incorporating one or more electrical resistance heating elements on the inner side of a heat-shrinkable tube in order to be capable of heating both the tube and a hot-melt adhesive. Electrical heating elements for heat-shrinkable tubes are also disclosed in the Japanese patent applications 58-009111, 58-009112, 58-009113, 60-061705, 7-248425, 58-023008, 54-118254. A problem existing in such protective packaging assemblies comprises the selection, when conducting an electrical current through a heating resistance element of a protective assembly for shrinking a protection sleeve, of the time when the heating is sufficient and the electrical current is to be interrupted.
It is an object of the invention to provide a protection assembly for optical fiber splices which has a reinforcing member and which can be easily produced.
It is another object of the invention to provide a protection assembly for optical fiber splices which can be easily used, not requiring extra control equipment.
In particular, a problem solved by the invention is how to reduce the risk of air being trapped inside a sleeve for protecting optical fiber splices and how to produce such a sleeve in a simple and reliable way.
Another problem solved by the invention is how to select, when conducting an electrical current through a heating resistance element of a protection assembly for shrinking a protection sleeve, the time when the heating is sufficient and the electrical current is to be interrupted.
Thus generally, a packaging assembly for protecting splices of optical fibers comprises as conventional an outer thin heat-shrinkable tube, preferably an inner, thicker heat-shrinkable tube, and a reinforcing rod. The reinforcing rod has an electrically conductive coating such as a metal layer on some surface, e.g. a flat surface, thereof. When heating the assembly for applying it tightly to a splice an electrical current is passed through the metal layer. Then heat is developed which causes the tubes to shrink. The metal layer can be applied to have a larger resistance in its central region where it then will be heated first, the shrinking of the tubes then also starting in the central region, as seen in the longitudinal direction of the tubes. This reduces the risk of forming trapped air bubbles. The metal layer can be made to act like an electrical fuse, being evaporated when the heat is sufficiently intensive and then interrupting the electrical current. This provides an easy and automatic operation of the assembly. This operation like an electrical fuse can obviously also be obtained for other forms of the heating resistance element different from a layer, for example by applying inside the tube or tubes any elongated element, such as a resistance wire, the element having a smaller cross-sectional area at its central portion or by using an elongated element made of a material having a resistance per unit which is larger in the central portion than in end portions thereof.
Applying a metal layer, e.g. a metal sheet cut a suitable shape, to a surface of the reinforcing rod, is a very simple operation. Obviously the resistance in the longitudinal direction of such a layer is also very easily controlled. For example the layer can easily be made narrower in its central portion where it will then have a higher resistance and thus be heated first and most intensely. The metal layer can also be printed on a flat surface and thereupon etched what will provide a very accurate control of the resistance of the various parts of the metal layer. Generally thus, in a central portion of such a layer of an electrically conductive material as seen in a longitudinal direction of the layer, the layer will then have a larger resistance per unit length as taken in the longitudinal direction of the layer than in end portions of the layer. Termed in another way, the layer of such an electrically conductive material generally has a substantially uniform resistivity and in order to obtain a more intense heating where required, it can thus have a smaller cross-sectional area in its central portion as seen in a longitudinal direction of the layer than in end portions of the layer.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.